A wide variety of IMDs for delivering a therapy or monitoring a physiologic condition which may employ one or more elongated electrical leads and/or sensors have been clinically implanted or proposed for clinical implantation in patients. Such IMDs may treat or monitor the heart, muscle, nerve, brain, and stomach or other organs. IMDs such as pacemakers and implantable cardioverter defibrillators (ICDs), are available for treating cardiac arrhythmias by delivering electrical impulses to the heart. Such devices sense the heart's intrinsic rhythm through cardiac leads carrying electrodes that may be implanted in the heart. When an abnormal rhythm is detected, which may be bradycardia, tachycardia or fibrillation, an appropriate electrical therapy is delivered to restore the heart's normal rhythm.
Leads associated with such IMDs typically include a lead body extending between a proximal lead end and a distal lead end and incorporates one or more exposed electrode or sensor elements located at or near the distal lead end. One or more elongated electrical conductors extend through the lead body from a connector assembly provided at a proximal lead end for connection with an associated IMD and an electrode located at the distal lead end or along a section of the lead body. Each electrical conductor is typically electrically isolated from any other electrical conductors and is encased within an outer sheath that electrically insulates the lead conductors from body tissue and fluids.
Implantable medical leads may extend from a subcutaneous implantation site of the IMD through an internal body pathway to a desired tissue site. The leads are generally preferred having small diameter, highly flexible, reliable lead bodies that withstand degradation by body fluids and body movements that apply stress and strain to the lead body and the connections made to electrodes. As lead bodies are made smaller and smaller and the number of lead conductors is increased or maintained, problems with lead insulation and integrity of lead conductors may become more prevalent.
Cardiac lead bodies are continuously flexed by the beating of the heart. Other stresses are applied to the lead body during an implantation or lead repositioning procedure. Movements by the patient can cause the route traversed by the lead body to be constricted or otherwise altered causing stresses on the lead body. At times, the lead bodies can be slightly damaged during surgical implantation, and the slight damage may progress in the body environment until a lead conductor fractures and/or the insulation is breached. The effects of lead body damage may progress from an intermittent manifestation to a more continuous lead related condition. In extreme cases, insulation of one or more of the electrical conductors may be breached, causing the conductors to contact one another or body fluids resulting in a low impedance or short circuit. In other cases, a lead conductor may fracture and exhibit an intermittent or continuous open circuit resulting in an intermittent or continuous high impedance.
Other problems can arise at the proximal lead end where the electrical connection between IMD connector elements and the lead connector elements may be intermittently or continuously disrupted, resulting in a high impedance or open circuit. Usually, such connector open circuit problems result from insufficient tightening of the connection mechanisms, such as a set screw, at the time of implantation followed by a gradual loosening of the connection until contact becomes intermittent or open or an incomplete lead pin insertion.
Such lead problems resulting in short or open circuits may be referred to, for simplicity, as “lead related conditions.” Typically, it is necessary for an attending clinician to diagnose the nature of a lead-related condition from available data, test routines, and patient symptoms. Then, it is necessary for the clinician to take corrective action, e.g., to either replace the lead, select different electrodes for sensing or pacing, or tighten the proximal connection. In severe cases, the lead-related condition may result in premature depletion of the battery energy of the IMD, requiring its replacement.
In the case of cardiac leads, the ability to sense an intrinsic heart rhythm accurately through a lead can be impaired by any of the above described lead related conditions. Complete lead breakage impedes any sensing functions, lead conductor fractures or intermittent contact can cause electrical noise that interferes with accurate sensing. Oversensing or undersensing can occur resulting in an incorrect interpretation of the heart rhythm by a pacemaker or ICD, potentially resulting in inappropriate withholding or delivery of electrical therapy. For example, oversensing may lead to the detection of tachycardia or fibrillation resulting in the inappropriate delivery of a high voltage shock therapy. Such therapy is painful to the patient and may be experienced repeatedly if a lead related condition is not diagnosed and corrected. Such inappropriate therapies deplete the ICD battery energy prematurely and could inappropriately induce ventricular fibrillation if delivered onto the T-wave.
During cardiac pacing or defibrillation, increased impedance of the stimulation path or the short circuit of lead conductors due to one of the above-described lead related conditions can reduce the effectiveness of a pacing or shocking below that sufficient to pace or defibrillate the heart. The failure of the delivered therapy can be dangerous to the patient and/or can necessitate applying further, higher energy, pacing or cardioversion/defibrillation shocks which can increase discomfort to the patient and is wasteful of battery energy.
The issue of the integrity of cardiac leads is a serious concern due to the potentially serious consequences to the patient. Certain pacemakers and ICDs have been provided with the capability of storing cardiac electrogram data prompted by the automatic determination of oversensing or undersensing of cardiac events, loss of effective pacing, out of range lead impedance measurements, etc. Such data can be telemetered to an external programmer when the physician interrogates the IMD and used by the clinician in troubleshooting any problems.
The lead impedance data and other parameter data is typically compiled and displayed on a monitor and/or printed out for analysis by the clinician. The clinician may undertake real time IPG parameter reprogramming and testing and observe the monitored surface ECG to try to pinpoint a suspected lead related condition that is indicated by the data and/or patient and/or device symptoms.
Certain external programmers that address the analysis of such data and symptoms include those disclosed in the following U.S. Pat. No. 4,825,869 (Sasmor et al.); U.S. Pat. No. 5,660,183 (Chiang et al.); and U.S. Pat. No. 5,891,179 (Er et al.), all incorporated herein by reference. The '869 patent describes processing a variety of uplink telemetered atrial and ventricular EGM data, stored parameter and event data, and the surface ECG in rule-based algorithms for determining various IPG and lead malfunctions. The '183 patent also considers patient symptoms in an interactive probability based expert system that compares data and patient systems to stored diagnostic rules relating symptoms to etiologies to develop a prognosis. The '179 patent discloses a programmer that can be operated to provide a kind of time varying display of lead impedance values in relation to upper and lower impedance limits. The lead impedance values are derived from pacing pulse current and voltage values and are either measured and stored in the IPG memory at an earlier time or comprise current, real time values that are uplink telemetered to the programmer for processing and display.
The diagnosis of lead related data at a later time in such ways is useful, but it is believed preferable to provide a more immediate response to a lead related condition by the IPG or monitor. The retrieved data may be suspect if a lead related condition causes the stored or real time telemetered data to be inaccurate. The physician may mistakenly rely upon such data to maintain or change programmed pacing parameters and modes, particularly if a lead related condition is intermittent and is not diagnosed.
Many proposals have been advanced to determine if a lead related condition has occurred and to modify the IPG operation and/or to provide a warning that is perceptible by the patient or can be telemetered to the external programmer when the physician interrogates the IPG or monitor. In addition, it has been a goal to automatically detect a lead conductor related condition and respond by switching pacing pathways to use available lead conductors that appear to be functioning properly. Prior art detection of lead related condition and various IPG responses to such detection are set forth in U.S. Pat. No. 4,140,131 (Dutcher et al.); U.S. Pat. No. 4,899,750 (Ekwall); U.S. Pat. No. 5,003,975 (Hafelfinger et al.); U.S. Pat. No. 5,137,021 (Wayne et al.); U.S. Pat. No. 5,184,614 (Collins); U.S. Pat. No. 5,201,865 (Kuehn); U.S. Pat. No. 5,224,475 (Berg et al.); U.S. Pat. No. 5,431,692 (Hansen et al.); U.S. Pat. No. 5,507,786 (Morgan et al.); U.S. Pat. No. 5,534,018 (Wahistrand et al.); U.S. Pat. No. 5,549,646 (Katz et al.); U.S. Pat. No. 5,722,997 (Nedungadi et al.); U.S. Pat. No. 5,741,311 (McVenes et al.); U.S. Pat. No. 5,755,742 (Schuelke et al.); and U.S. Pat. No. 5,814,088 (Paul et al.). All of these patents are incorporated by reference.
Most of these patents disclose systems for periodically measuring lead impedance and comparing the impedance measurements with upper and lower impedance values or ranges and either storing the data for later retrieval, and/or changing a pacing or cardioversion/defibrillation path, and/or adjusting the delivered pacing energy, and/or alerting the patient by generating sound or stimulation warning signals. Most of the above-incorporated patents depend on the generation of an impedance reading during a period of time when the pacemaker is not providing a stimulation pulse to the heart or, alternatively, sample and hold some portion or portions of a pacing or defibrillation signal, digitize some characteristic or characteristics inherent in that signal, and have that digitized signal processed by an on-board algorithm or circuit in order to produce an impedance value for the conductor under test. The impedance value is typically compared to upper and lower impedance thresholds or impedance reference value, and employed as described above. In most cases, event data comprising the signal value and time and date are stored in memory whenever the impedance value exceeds or falls below the upper and lower impedance thresholds (i.e., the lead impedance is out of range). Certain of the above-incorporated patents, e.g. the '786 patent, also provide monitoring and storage of other parameters of IPG operation, e.g., battery voltage, for later retrieval and analysis by a clinician in an uplink telemetry session. Others of the above-incorporated patents disclose some processing of the lead impedance values within the IPG, and storage of the processed data for later retrieval and analysis by the clinician. The above-incorporated '975 patent discloses measuring unipolar and bipolar lead impedances, incrementing an error counter at least when the bipolar lead impedance value is out of range, and switching to a unipolar lead configuration, if one is available that exhibits a lead impedance value that is in the acceptable impedance range. The above-incorporated '750 patent discloses measuring output energy delivered during a pacing pulse, deriving a lead impedance value therefrom that is compared to a moving average impedance value, and incrementing a first error counter if a series, e.g., three, of such lead impedance values are out of range. In addition, characteristics of sensed heart signals are monitored, and the count of a second error counter is incremented if a series of the sensed heart signals exhibit an abnormality, e.g. an abnormal slew rate that could be due to a lead related condition. The counts are interrogated and displayed by an external programmer in an uplink telemetry session to alert the clinician of a possible lead related condition that should be investigated.
The '742 patent discloses an ICD lead impedance measurement system that measures impedance of all of the cardioversion/defibrillation leads and pacing leads using three leads at a time. A force lead and a measure lead are selected to drive current through a lead under test and to measure the voltage induced in the lead under test. Lead impedance values are derived and compared to upper and lower impedance thresholds. Out of range lead impedance value data causes an invalid flag to be set, may cause a patient warning to be emitted, and is stored as event data for later interrogation and uplink telemetry to the external programmer. The uplink-telemetered data is applied to sets of impedance rules for determining short circuit and open circuit lead related conditions. It is suggested that these rules and the testing process could be incorporated into the IPG to set a flag that identifies the lead defect and to emit a patient alert.
U.S. Pat. No. 6,317,633, issued to Jorgenson et al., incorporated herein by reference in its entirety discloses a self-testing system providing a lead status report that identifies particular lead-related condition for each lead employed in an IMD based on comparisons of periodic lead impedance measurements to upper and lower limits and loss of capture values. Optionally, such a monitor would cause a patient warning to be emitted and enable the IMD to alter its operating mode or to discontinue using a defective lead.
Comparison of a lead impedance measurement taken at a particular point in time to a fixed range of acceptable values or a fixed reference value can be useful in detecting a lead-related condition that has already manifested itself as an extremely high or extremely low impedance. Setting a fixed range, however, does not allow gradually occurring lead conditions to be detected early on. Defining a fixed range more narrowly in order to detect a lead condition earlier may result in undesired false positive detections causing a clinician to spend time investigating a problem that may not exist. A lead-related condition that is gradually worsening over time may still affect lead and IMD performance. Such conditions are preferably caught early to prevent clinical manifestation of the problem. Therefore, it is desirable to monitor trends of lead impedance changes so that a gradually occurring condition may be detected early on. Furthermore, recognition of the time course of the development of a lead-related condition may be important in diagnosing the cause and allowing prompt, appropriate corrective action. The above cited '750 patent addresses this issue in part by determining a moving average of a measured impedance and counting deviation from the norm.
Specific types of lead-related conditions may be associated with certain types of lead designs. For example, degradation of insulation between conductors may be specific to certain types of leads having coiled conductors arranged coaxially within the lead body, isolated from each other by an intervening insulating layer. After chronic exposure to the considerably hostile environment within the human body, the middle layers of insulation may break down between the conductors within the lead body. Metal ionized oxidation of the middle layers is thought to be the mechanism behind this type of middle insulation degradation which allows the infiltration of body fluids to create a short between two conductors running coaxially. The gradual degradation of the middle insulating layer results in a gradual decrease in sub-threshold impedance measured between the two electrodes associated with the two shorted conductors. This phenomenon has been observed between the ring electrode conductor and the coil electrode conductor in true bipolar cardiac defibrillation leads. Because the ring electrode is generally used for sensing the heart's intrinsic rhythm, a short between the ring electrode conductor and the coil conductor may produce oversensing and result in inappropriate therapy deliveries. Measurement of the impedance between the ring and coil electrodes show a decline, however, this decreased impedance could also be the result of an outer insulation breach.
Therefore, in some situations, a single lead-impedance measurement may not be adequate to specifically diagnose a lead-related condition. This problem is partially addressed by U.S. Pat. No. 5,944,746, issued to Kroll, incorporated herein by reference in its entirety. A system is disclosed for periodically obtaining a lead impedance measurement from a pacing tip to a high voltage shocking coil. The impedance is compared to a previously obtained measurement to determine if the impedance has increased. The system is further adapted to compare the impedance measurement to the impedance measured between the pacing lead and the casing of the implantable device to determine whether any increase in the measured impedance is due to a problem with the pacing lead or a problem with the high voltage coil or high voltage lead.
Since problems associated with lead-related conditions may be intermittent and are not routinely encountered in all patients, the task of recognizing and trouble-shooting lead-related conditions can be challenging to the physician. What is needed, therefore, is an automated method for detecting a lead-related condition based on trends in lead impedance measurements, which may include comparisons of measurements made along multiple conductive pathways. Furthermore, it is desirable that detection of lead-related conditions occurs prior to clinical manifestations that may pose risk to the patient. Reliable diagnosis of lead-related conditions will allow a physician to make prompt corrective actions with confidence and may allow an implantable device to make automated corrective actions.